As the manipulation of sound in virtual space begins to attract people's attention, audio sound, especially 3D audio sound, becomes more and more important in providing an artificial sense of reality, for instance, in various game software and multimedia applications in combination with images. Among many effects that are heavily used in music, the sound field effect is thought of as an attempt to recreate the sound heard in a particular space.
In this context, 3D sound, often termed as spatial sound, is understood as sound processed to give a listener the impression of a (virtual) sound source at a certain position within a three-dimensional environment.
An acoustic signal coming from a certain direction to a listener interacts with parts of the listener's body before this signal reaches the eardrums in both ears of the listener. As a result of such an interaction, the sound that reaches the eardrums is modified by reflections from the listener's shoulders, by interaction with the head, by the pinna response and by the resonances in the ear canal. One can say that the body has a filtering effect on the incoming sound. The specific filtering properties depend on the sound source position (relative to the head). Furthermore, because of the finite speed of sound in air, the significant inter-aural time delay can be noticed, depending on the sound source position. Here Head-Related Transfer Functions (HRTFs) come into play. Such Head-Related Transfer Functions, more recently termed the anatomical transfer function (ATF), are functions of azimuth and elevation of a sound source position that describe the filtering effect from a certain sound source direction to a listener's eardrums.
An HRTF database is constructed by measuring, with respect to the sound source, transfer functions from a large set of positions to both ears. Such a database can be obtained for various acoustical conditions. For example, in an anechoic environment, the HRTFs capture only the direct transfer from a position to the eardrums, because no reflections are present. HRTFs can also be measured in echoic conditions. If reflections are captured as well, such an HRTF database is then room-specific.
HRTF databases are often used to position ‘virtual’ sound sources. By convolving a sound signal by a pair of HRTFs and presenting the resulting sound over headphones, the listener can perceive the sound as coming from the direction corresponding to the HRTF pair, as opposed to perceiving the sound source ‘in the head’, which occurs when the unprocessed sounds are presented over headphones. In this respect, HRTF databases are a popular means for positioning virtual sound sources.